The solution according to the invention is used for measuring a surface property of paper or paperboard. The paper or paperboard surface is illuminated and the illuminated paper or paperboard surface is imaged by a camera for measuring.
The properties used for describing the surface properties of paper and paperboard include roughness and gloss which are used e.g. for estimating the printability of paper. The probably most common way of measuring paper roughness is based on airflow measuring methods which are employed in the apparatuses of Bendtsen, Parker-Prit-Surf-Sheffield, Bekk and Gurley Hill, for example. In these solutions a cylindrical element is pressed against the paper or paperboard surface with a certain amount of force. A pressure difference with respect to the ambient air is caused in the element, and the air flowing through an opening between the edge of the cylindrical element and the paper or paperboard surface is measured. There are several problems related to these measurements. The parameter representing roughness is also influenced by the porosity of paper. The measurement is also integrally directed at a large surface area, for which reason accurate information on the detailed structure of the paper surface cannot be obtained. This is disadvantageous to the estimation of printability, for example. A further disadvantage is an error factor caused by the dependency between the measuring result and the force used for pressing the cylindrical element against the paper or paperboard surface.
The surface properties of paper and paperboard can also be measured optically. In profilometric roughness measurement the paper surface is examined unidimensionally using a focused ray in the same way as when reading a CD. Problems are caused by porosity, paper transparency and strong reflectance, if any, which distort the profile. Unidimensionality of the measurement is also a disadvantage. Roughness can also be measured utilizing light scattering. In the prior art solution the paper surface is conventionally illuminated at an oblique angle (45xc2x0 to 85xc2x0) with respect to the normal of the surface, and the intensity distribution of radiation from the direction of specular reflection is measured to determine roughness. This solution is described in greater detail e.g. in Finnish publication no. 56453, which is incorporated herein by reference. Specular reflection is typically also measured in connection with the measurement of the gloss. In the standardized solution according to the prior art, gloss of paper or paperboard is measured by illuminating the paper or paperboard surface at a large angle (e.g. 75xc2x0) with respect to the surface normal and detecting, optical radiation reflected from the surface at the angle of reflection. This solution is described more closely in standard T 480 om-92, which is incorporated herein by reference. The prior art measuring arrangements measure gloss and roughness integrally from a large area the size of which is usually dozens of square millimeters. The average measured from a large area does not give accurate information on the structure of the paper surface and thus e.g. printability cannot be estimated reliably. The large measuring angle further impairs the measuring accuracy. An additional problem is that the surface roughness and the surface gloss are mixed with each other and they cannot be determined separately with any certainty.
An object of the invention is to provide a method and an apparatus implementing the method to eliminate the above-mentioned problems. This is achieved with the solution according to the invention which relates to a method of determining a surface property of paper or paperboard, the method comprising illuminating the paper or paperboard surface and imaging the illuminated paper or paperboard surface onto the pixels of a camera""s detector surface. For measuring at least roughness as the micro surface property, the method according to the invention comprises illuminating the paper or paperboard surface with collimated light substantially parallel with the normal of the paper or paperboard surface; controlling the reflected optical radiation with an aperture of the desired size before the camera; producing an image of the paper or paperboard surface onto the pixels of the camera""s detector surface by means of the reflected optical radiation, each of the pixels imaging an area in the order of micrometers of the paper or paperboard surface, and measuring at least the roughness of paper or paperboard surface on the basis of the intensity of the pixels on the camera""s detector surface.
The invention also relates to a measuring arrangement for determining a surface property of paper or paperboard, the measuring arrangement comprising an optical power source for illuminating the paper or paperboard surface and a camera for imaging the illuminated paper or paperboard surface onto the pixels of the camera""s detector surface. The measuring arrangement according to the invention comprises, for measuring at least one micro surface property, a collimating optical block for collimating the optical radiation emitted from the optical power source; a partially permeable mirror for projecting the optical radiation onto the paper or paperboard surface substantially parallel with the normal of the paper or paperboard surface; an imaging optical block for producing an image of the illuminated paper or paperboard surface onto the pixels of the camera""s detector surface by means of the reflected optical radiation; and an aperture of the desired size for controlling the reflected optical radiation; the measuring arrangement being arranged to measure at least roughness of the paper or paperboard surface on the basis of the intensity of the pixels on the camera""s detector surface.
The dependent claims disclose the preferred embodiments of the invention.
The solution according to the invention is based on illuminating the paper or paperboard surface with collimated optical radiation from the direction of the surface normal. An aperture is used for controlling the quality of the micro surface property to be measured. When roughness is measured, the aperture is used for adjusting the depth of focus of the image formed onto the pixels of the camera""s detector surface. When gloss is measured, the aperture is used for adjusting the direction of optical radiation entering the camera. In both cases the micro surface properties are measured by means of the intensity received by the pixels on the camera""s detector surface.
The measuring arrangement and method according to the invention provide several advantages. The invention enables measuring of a surface property of paper or paperboard from a surface area in the order of micrometers, in which case the surface properties can be determined as micro surface properties. This allows to separate roughness and gloss reliably from each other. Thanks to this, the printability of paper or paperboard can be estimated accurately, which results in considerable savings in the production costs of various printed goods because paper or paperboard processing can be controlled during the manufacture. In practice this means that the consumption of ink used in printing, for example, can be optimized, which allows to guarantee good quality of the text and pictures.